This invention relates to producing modulated gradient fields in magnetic resonance imaging (MRI) systems.
A variety of modulated gradient field waveforms are known to be useful in MRI systems. Some MRI systems, such as those disclosed in Rzedzian U.S. Pat. No. 4,628,264, produce a sinusoidally varying gradient field by sinusoidally driving a circuit comprising the gradient coil connected in parallel with a capacitor. The desired current is realized by selecting a capacitor/coil combination with a resonant frequency corresponding to the desired gradient field frequency, and driving the coil at that frequency with a sinusoidal voltage source. In this way, the voltage source supplies only a small fraction (1/Q, where Q is the quality factor of the resonant capacitor/coil circuit) of the coil current. The majority of the coil current comes from the resonant discharge of the capacitor.
Other MRI systems produce a steady gradient field by driving the gradient coil with a pulse of constant amplitude. The leading and trailing edges of these pulses rise and fall gradually, consistent with limits on the rate at which the current through the gradient coil can be changed. For example, trapezoidal pulses of coil current are often used.